Gas turbine engines (GTEs) produce power by extracting energy from a flow of hot gas produced by combustion of fuel in a stream of compressed air. In general, GTEs have an upstream air compressor coupled to a downstream turbine, with a combustion chamber (combustor) in between. The flow of hot gas is produced when a mixture of compressed air and fuel is burned in the combustor, In typical GTEs, multiple fuel injectors direct the fuel to the combustor for combustion.
Combustion of typical fuels results in the production of some undesirable constituents, such as NOx, in GTE. exhaust emissions. Air pollution concerns have led to government regulations that limit the emission of NOx in GTE exhaust, One method used to reduce NOx emissions of GTEs is to use a well-mixed lean fuel-air mixture (a fuel-air mixture having a lower fuel to air ratio than the stoichiometric ratio) for combustion in the combustor. However, in some cases, using a lean fuel-air mixture may make combustion unstable. To provide a stable flame while meeting NOx emission regulations, some fuel injectors direct separate streams of a lean fuel-air mixture and a relatively richer fuel-air mixture to the combustor. The lean fuel-air mixture may provide low NOx emissions, while the richer fuel-air mixture may provide flame stabilization.
In some cases, the fuel injector may also be configured to direct both a liquid and a gaseous fuel to the combustor. Such a fuel injector, called a dual fuel injector, may enable the GTE to operate using both liquid fuel (such as, for example, diesel) and gaseous fuel (such as, for example, natural gas), depending upon the conditions and economics of any particular GTE operating site. In dual fuel injectors, the liquid or the gaseous fuel may be directed to the fuel injector, mixed with air, and delivered to the combustor. Such a dual fuel injector may include both liquid fuel supply lines and gaseous fuel supply lines, along with suitable valves that enable the liquid fuel supply to the injector to be switched off while the GTE is operating on gaseous fuel, and the gaseous fuel supply to the injector to be switched off while the GTE is operating on liquid fuel. That is, a GTE's dual fuel injectors selectively direct either gas or liquid fuel to the combustor at any time.
U.S. Pat. No. 8,099,940 B2 ('940 patent) to Twardochieb et al, (issued Jan. 24, 2012) discloses a dual fuel injector for a gas turbine engine that includes both a main fuel stream and a pilot fuel stream. The pilot fuel stream includes a fuel-air mixture that is richer in fuel relative to the main fuel stream. The lean fuel-air mixture directed into a combustor burns to produce a low temperature flame, The NOx emissions of the GTE operating on the lean fuel-air mixture is low. The richer fuel-air mixture, directed to the combustor as the pilot fuel stream, burns at a higher temperature and serves to stabilize the combustion process at the cost of slightly increased NOx emissions. To reduce NOx emissions while maintaining the stability of the combustion process, a control system of the GTE increases the flow of pilot fuel-air mixture during startup or when an unstable combustion event is detected. The '940 patent further discloses that a dual fuel injector may comprise an air assist circuit terminating in an air assist nozzle, which supplies externally pressurized air while the GTE is starting on liquid fuel. Whereas a running GTE may utilize compressed air generated by the GTE's compressor to obtain a suitable air-liquid fuel mixture for the pilot, such compressed air may not be available during GTE startup. Thus, during GTE startup, air assist provided by the air assist circuit may be advantageously used to atomize (or improve atomization of) the liquid fuel in the pilot fuel stream, and thereby create (or improve) the rich fuel air mixture directed into the combustor by the pilot. It may be noted that the externally-pressurized air assist is of much lower pressure than the compressed air normally generated by the GTE's compressor.
When a GTE runs on gaseous fuel, it is somewhat typical that the flows of pilot gaseous fuel from respective dual fuel injectors will differ due to variations in manufacture (within manufacturing tolerances). This variability of pilot gaseous fuel flows can cause inefficient combustion of the pilot (and main) gaseous fuel, resulting in excessive NOx emissions or engine knock. It is known in the art that restricting the pilot gaseous fuel flow from each respective dual fuel injector substantially reduces the variability in the respective gaseous fuel flows. This is typically accomplished by, for example, placing a restrictor, such as an orifice or a restricting nozzle, at the end of the pilot gaseous fuel line. Additionally, a restriction—such as an orifice—placed at the end of the pilot gaseous fuel line may serve to prevent the flow of fuel or combustion byproducts into the pilot gaseous fuel line when the pilot gaseous fuel line is not in use. Although the magnitude of restriction need not be large to reduce the variability of pilot gaseous fuel flows among respective fuel injectors, the restriction necessarily results in a pressure drop.
Although the inclusion of air assist circuits may improve performance of a GTE, the air assist circuits disclosed in the '940 patent can also be problematic. In particular, their inclusion may complicate the design, manufacture, and maintenance of a GTE, resulting in additional labor and material costs. Moreover, inclusion of air assist circuits can present a risk that fuel or byproducts of fuel combustion, such as sulfur residue, may flow into the air assist circuit assembly when air assist is not being used. That is, the pressures generated by combustion or by fuel injection may cause an undesirable reverse flow of fuel or combustion byproducts to enter the air assist circuit, The presence of fuel or combustion byproducts in the air assist assembly may clog or corrode the air assist assembly, undermining safe, efficient operation of a dual fuel injector.
The disclosed dual fuel injector is director toward overcoming one or more of the problems set forth above and/or other problems of the prior art.